The present invention relates to a coated cemented carbide insert (cutting tool), particularly useful for milling of stainless steels and super alloys but also milling of steels in toughness demanding applications.
During milling of various materials with coated cemented carbide cutting tools, the cutting edges are regarded as being worn according to different wear mechanisms. Wear types such as chemical wear, abrasive wear and adhesive wear, are rarely encountered in a pure state, and complex wear patterns are often the result. The domination of any of the wear mechanisms is determined by the application, and is dependent on both properties of the work piece and applied cutting parameters as well as the properties of the tool material. At high cutting speeds, the amount of heat generated in the cutting zone is considerable and a plastic deformation of the cutting edge may occur, which in turn yields an enhanced wear by other mechanisms. The tool life is also often limited by edge chipping caused by so-called comb cracks that form perpendicular to the cutting edge. The cracks originate from the varying thermal and mechanical loads that the cutting edge is subjected to during the intermittent cutting process. This is often even more evident in machining with coolant, which enhances the thermal variations.
Many stainless steels in general, and super alloys in particular, possess material properties that are most unfavorable from a machinability point of view, when compared to steels. The low thermal conductivity of these materials results in high temperatures in the tool and work piece contact zones. The use of coolant to reduce the tool temperature can give very large thermal variations and induce high internal tool loads. These work piece materials are also often prone to deformation hardening. Deformation hardening of the work piece in the cutting zone can result in high cutting forces, or even cutting in a hardened layer, which both will lead to accelerated tool wear. Furthermore, most stainless steels and superalloys show good adherence towards many coating materials and adhesive wear and rapid chipping of the tool edge due to re-cutting of chips can be substantial. It is thus clear that coated cemented carbide tools intended for use in milling of stainless steels and super alloys must have properties often extending those that are generally needed for tool materials.
In general, measures can be taken to improve the cutting performance with respect to a specific wear type. However, very often such actions will have a negative effect on other wear properties and successful tool composite materials must be designed as careful optimisations of numerous properties. A simple measure to increase the toughness often needed for milling tools is to increase the binder phase content. However, this will also quickly reduce the wear resistance and also the resistance to plastic deformation. Addition of cubic carbides such as TiC, TaC and NbC is another mean to influence the tool properties, and large additions in combination with high binder phase contents can give relatively good toughness behavior. However, this addition can have a negative influence on comb crack formation and edge chipping tendencies.
Since it is obviously difficult to improve all tool properties simultaneously, commercial cemented carbide grades have usually been optimised with respect to one or few of the above mentioned wear types. Consequently, they have also been optimised for specific application areas.
U.S. Pat. No. 6,062,776 discloses a coated cutting insert particularly useful for milling of low and medium alloyed steels and stainless steels with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces under unstable conditions. The insert is characterised by a WC-Co cemented carbide with a low content of cubic carbides and a rather low W-alloyed binder phase and a coating including an innermost layer of TiCxNyOz with columnar grains and a top layer of TiN and an inner layer of κ-Al2O3.
U.S. Pat. No. 6,177,178 describes a coated milling insert particularly useful for milling in low and medium alloyed steels with or without raw surface zones during wet or dry conditions. The insert is characterised by a WC-Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiCxNyOz with columnar grains, an inner layer of κ-Al2O3 and, preferably, a top layer of TiN.
U.S. Pat. No. 6,250,855 discloses a coated milling insert for wet or dry milling of stainless steels of different composition and microstructure. The coated WC-Co based cemented carbide inserts includes a specific composition range of WC-Co without any additions of cubic carbides, a low W-alloyed Co binder and a hard and wear resistant coating including a multilayered structure of sub-layers of the composition (TixAl1-x)N.
EP 1103635 provides a cutting tool insert particularly useful for wet and dry milling of low and medium alloyed steels and stainless steels as well as for turning of stainless steels. The cutting tool is comprised of a cobalt cemented carbide substrate with a multi-layer refractory coating thereon. The substrate has a cobalt content of from about 9.0 to about 10.9 wt % and contains from about 1.0 to about 2.0 wt % TaC/NbC. The coating consists of an MTCVD TiCxNyOz layer and a multi-layer coating being composed of κ-Al2O3 and TiCxNyOz layers.
It has now been found that enhanced cutting performance can be obtained by combining many different features of the cutting tool. The cutting insert has excellent performance preferably for milling of stainless steels and super alloys, but also for steels in toughness demanding operations. At these cutting conditions, the cutting tool according to the invention displays improved properties with respect to many of the wear types mentioned earlier.